This invention relates generally to screw anchors for fastening fixtures and other objects to both hollow and solid walls, and more particularly to a removable and reusable plastic anchor constituted by a socket and a diamond-shaped shank extending therefrom, the shank being compressible to form a tongue that is insertable in a hole drilled in the wall to accommodate the socket.
In hollow walls formed of plaster board, "Sheet-rock," plywood and other relatively thin panel material, one is often faced with the problem of mounting on the wall drapery transverse rods, lighting fixtures and other objects which impose a heavy stress on the wall. Such walls are usually incapable of retaining standard screws, and it is for this reason that an anchoring socket of some sort is required in order to secure a screw to the wall to afford a strong and reliable fixture installation.
The nature of the anchoring socket must be such that it is adapted to engage the inner surface of the hollow wall so that the socket cannot be pulled out and will not twist when the screw is inserted and taken up to secure the fixture to the wall.
One commonly used screw anchor makes use of a metal sleeve which is longitudinally split to create parallel ribs extending between a flanged front socket received in a hole drilled in the wall and a rear socket, the rear socket being threaded. The arrangement is such that when a screw is inserted to engage the rear socket, turning of the screw causes the ribs to deform and to flare out until they are drawn flush with the interior surface of the wall. The flaring of the ribs spreads the load over a relatively wide area of the wall adjacent the hole drilled therein.
One major objection to a metal screw anchor of this type is that there is no efficient way of removing the anchor from the wall; for once the ribs are deformed to their flared condition, they cannot be straightened. Hence to remove the screw anchor, one must break the flange off the front socket and push the remainder of the screw anchor into the wall. This not only destroys the screw anchor so that it cannot be reused, but damage is often done to the wall in an attempt to remove the flange.
To overcome the drawbacks of conventional screw anchors, the patent to Kapnek, U.S. Pat. No. 3,487,746 discloses an anchor formed by parallel ribs which are pivotally secured to front and rear sockets, a spring causing the ribs to pivot and to be held tight against the interior of the wall. The anchor can be straightened out by the insertion of a screw to expand the spring, thereby permitting the withdrawal of the anchor from a hole made in the wall. While an anchor of this type is removable and reusable, its relatively complex structure makes the manufacture and assembly thereof expensive. Moreover, an anchor of the Kapnek type is not usable in solid walls.
Dwyer, in his U.S. Pat. No. 3,213,746, discloses a less costly screw anchor which is molded of plastic material. The anchor includes a flexible web member with a wish-bone-like shape which, when a screw is inserted, spreads apart to hold the socket of the anchor in the wall. In this arrangement, the inner surface of the wall is engaged only by teeth formed on the sloped surfaces of the webs in a small area surrounding the hole, so that the load is concentrated in this area rather than over the broad area required to distribute the load. Such load distribution is essential with relatively weak plaster board and the like; for otherwise a heavy load imposed on the anchor will pull the anchor out of the hole.
Still another approach is that disclosed in the McSherry U.S. Pat. No. 3,651,734 in which a screw socket received in the hole in the wall is coupled to a toggle lock. When collapsed, this toggle is insertable through the hole. When expanded to an over-center stable position, the toggle acts to lock the anchor within the hole. One serious drawback of this anchor arrangement is that once installed it cannot be easily removed.